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Chapter 1: Introduction

1.1. Background: Evidence for climatic shifts in mountain vegetation

The global climate has shown an increase of 0.6°C over the last century and

is predicted to rise between 1.5°C and 5.8°C over the next century (IPCC

2001). Ecosystems at high altitudes and latitudes are expected to be

particularly sensitive to climatic change (Pauli et al. 1996). A number of

models, using a climate envelope approach, has predicted reductions in

species distribution for arctic/alpine plants due to climate change (e.g.

Huntley et al. 1995, Cannell et al. 1997, Harrison et al. 2001, Bakkenes et al.

2002, Berry et al. 2002, Pearson et al. 2002, Thomas et al. 2004), due to loss

of suitable climatic conditions. For example, over recent years there has

been increasing evidence that plant species in mountain areas are being

found at higher altitudes than previously (Grabherr et al. 1994, Keller et al.

2000, Kullman 2002, Klanderud & Birks 2003, Peñuelas and Boada 2003,

Sanz-Elorza et al. 2003). Some of these European studies showed an

upward movement of plant species, which has been attributed to climate

change (Grabherr et al. 1994, Kullman 2002, Klanderud & Birks 2003).

The Alps

Grabherr and colleagues surveyed 26 summits of over 3,000 m above sea

level (a.s.l) in the central Alps (western Austria and eastern Switzerland) in

the summer of 1992. The data from this survey were then compared with

historical records of cover and abundance of vascular plant species

(Grabherr et al. 1994, 1995, Pauli et al. 1996, Gottfried et al. 1998). This

showed an upward movement in the distribution of plant species. Working on

the basis of 12 very precise historical records, they calculated the rate of

upward movement of the nine fastest moving plant species to be between 1 -

4 m per decade, with the rest extending their upward distribution by less than

one meter per decade (Grabherr et al. 1994). The historical records used for

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these calculations were collected between 1900 and 1920. The greatest

increases in species richness were most pronounced at lower altitudes

(Grabherr et al. 1994).

Further west in the Alps, Keller and colleagues were able to study a “unique

70 year data set of high-elevation permanent vegetation plots” in Schynige

Platte in the Bernese Alps of Switzerland (Keller et al. 2000). They found that

the probability of finding plant species which favour warmer conditions had

increased since the 1930s, and they found a decrease in the number of

species which prefer cooler conditions. These studies took place at altitudes

above those where direct anthropogenic influences, such as livestock

grazing, would have an effect (Keller et al. 2000). Also the change was found

to be independent of changes in availability of nutrients, and so they

concluded that changes in climatic conditions explained the changes seen in

the vegetation. Their observations show close agreement with the upward

migrations reported by Grabherr and colleagues.

Spain

Sanz-Elorza et al. (2003) investigated ’significant changes in vegetation’ over

a 40 year period in the Peñalara massif (2430 m a.s.l) in central Spain. They

found an upward movement of shrubs from lower levels into the Cryoro-

Mediterranean sub-alpine zone. This movement is possibly due to increases

in temperature, changes in monthly rainfall patterns and reductions in snow

cover. However other factors were also seen as being important, such as

changes to the grazing regime in the 19th century.

In a similar study in North East Spain, Peñuelas and Boada (2003) showed a

“progressive replacement of cold-temperate ecosystems by Mediterranean

ecosystems”, with a ca. 70 m upward altitudinal shift of beech (Fagus

sylvatica). Here again changes in grazing regime and shepherding practice,

as well as climatic change, have had an effect on the observed shifts in

vegetation zones (Peñuelas and Boada 2003). This shows that the

relationship between shifts in vegetation zones and climate change can be

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part of more complex interactions, which can include changes in land

management practice.

Scandinavia

Similar upward movements of plant species have been observed by

Klanderud and Birks (2003) in the Jotunheimen mountains of central Norway

(61° N) at altitudes between 1500 m and the summits above 2000 m. They

found an increase in species richness and distribution on 19 out of 23

mountains surveyed in 1998, when compared with detailed surveys carried

out in 1930 – 31. No details are given about the four mountains where they

do not report increases in species richness or distribution. The greatest

increases were recorded in the east and at lower altitudes, with only slight

changes in richness on windswept summits. A few species were reported to

have decreased in frequency, these were species associated with late lying

snow beds. However, Klanderud and Birks attribute this change to the effects

of acid deposition, rather than climate change. They also report that some of

the species most frequently found at high altitudes have “disappeared” from

sites at lower altitude where they were previously recorded, but have

increased in abundance at the highest altitudes. This they suggest may be

due to the effects of strengthening competition. They considered the possible

influence of other factors such as changes in grazing pressure, nitrogen

deposition, and tourism, along with their interactions, but concluded climatic

warming to be the major driver for the upward shifts of vegetation which they

have reported (Klanderud & Birks 2003).

In the Scandes mountains of northern Sweden (63°26’ N 13°06’ E), Kullman

(2002) used a combination of repeat surveying of historical records from

1955, and age determination of saplings to investigate changes in the range-

margin of seven tree and shrub species. He found individual species moving

between 120 and 375 m in altitude and a rise in the overall forest tree line of

between 100 m to 150 m. By examining the age structure of the saplings,

Kullman determined that they had established since 1987. This coincides

4

with a period of warm summers and mild winters in that area (Kullman 2002).

The saplings observed showed unchecked height increments, which is not

characteristic of exposed high altitude saplings. Normally such saplings

would exhibit stunted growth due to repeated winter dieback of the shoots

(Grace 1997). It is also notable that dwarf shrubs such as Vaccinium

myrtillus, Empetrum hermaphroditum and Betula nana did not extend their

altitudinal ranges in the same manner. In contrast, in the more oceanic

Jotunheimen mountains of Norway, Klanderud and Birks (2003) did find V.

myrtillus had expanded its altitudinal range. This suggests that not all plant

species will respond to global climatic change in the same way and at the

same rates between and within sites.

Australia

It has been difficult to find similar studies from mountain areas outside

Europe. Interestingly, a study from Australia has shown a downward

movement of alpine vegetation. Kirkpatrick and co-workers, by analysing

photographs taken between 950 m and 1014 m a.s.l. in 1989 and 2000 on

the summit of Hill One, in the south of Tasmania, found that sub alpine

“bolster heath” had retreated down slope and was being replaced by “Alpine

heath” plants. This downward retreat of the bolster heath was attributed to

increased exposure to south-westerly winds in winter (Kirkpatrick et al. 2002).

Tasmania, being an island, is subject to strongly oceanic climatic conditions

which are much closer to Scottish conditions than the Alps.

Scotland

Unfortunately, in Scotland there is little evidence of the impact of climate

change on the altitudinal range of plant species. This is largely due to

relatively few early published botanical surveys of the Scottish Highlands

providing reference points. The first systematic botanical survey of the

Scottish uplands was not carried out until the turn of the 19th and 20th

centuries by Robert Smith (Smith 1900). However, there is insufficient detail

5

given in his published survey of the “North Perthshire District” (Smith 1900)

for it to be possible to carry out a re-survey with any confidence.

Unfortunately, details of Smiths own notes and records are unavailable. The

next available historical botanical survey in the Highlands comes from Watt

and Jones (1948), who were part of the Cambridge Botanical Expedition to

the Cairngorms which took place over short periods during the summers of

1938 and 1939 (Watt & Jones 1948). The expedition based itself at

Glenmore Lodge and the main survey area was between Coire Laogh Mór

(north east) and Creag an Leth-choin (south west). The most detailed

surveys were carried out in Coire na Ciste (referred to as Margaret’s Coffin)

and Coire Cas. Unfortunately these areas were developed in the 1960’s to

form the core of the Cairngorm ski area (for a description of changes in

vegetation cover in Coire Cas between 1966 and 1987, see Bayfield 1996).

Watt’s personal notes and records along with photographic plates are now

archived in the Library at The University of Aberdeen. While these provide

good botanical detail of areas outwith those affected by the skiing

development, i.e. Ben Macdui and Braeriach, the spatial detail given is

insufficient for a re-survey to be undertaken with confidence.

In the 1950s a systematic botanical survey of the Scottish Highlands was

carried out McVean and Ratcliffe (1962). Unfortunately detailed records of

this survey were not available.

Given this lack of long term botanical records, it is not surprising that there

are no published studies of re-surveying historical surveys in Scotland. There

is only one study showing an upward shift in vegetation. Bayfield and

colleagues (1998) have reported that, over a 40 year period, Pinus sylvestris

saplings have colonised areas up to 850 m a.s.l. in the Northern Corries of

the Cairngorms. Following observations made by Pears (1967), they

recorded the density of these saplings along fixed transects in 1984 and

1994, and found that there had been an increase in density in the second

survey. The highest densities of saplings were found on the ridges between

the corries at 650 m in the ski area. These findings are similar to those

reported by Kullman (2002) in Sweden, however it is unclear whether this

6

increase in saplings is due to climatic change or the reduction in grazing

pressure from red deer (Cervus elaphus) (Bayfield et al. 1998).

The European studies detailed above are from areas which have a higher

continentality index than Scotland (Figure 1.1). Continentality is a measure of

how the climate of an area is affected by its remoteness from the oceans and

oceanic air (Conrad 1964). The most often quoted indicator is the difference

between average prevailing January and July temperatures.

Figure 1.1. Conrad’s index of continentality based on European Gridded Climatology for 1961 -1990 (Hulme et al. 1995), taken from Crawford (2000). Areas nearest the oceans with low continentality have a low annual temperature range and those further away have a high annual temperature range.

While the Jotunheimen mountains of Norway are climatically closer to

Scotland than the Alps, they are still more continental than the Grampian

mountains in Scotland. The results of Klanderud and Birks (2003) show a

distinct trend with increasing change in species and species richness from

west to east, suggesting that continentality may be a factor. Unfortunately,

they did not have sufficient data to investigate this fully (Klanderud pers.

comm.). This would suggest that predictions for Scotland based on these

studies should be treated with caution, as the Scottish climate has a strong

oceanic rather than continental influence. With the changes in climate

predicted with global warming, the western and northern regions of Europe

will become increasing oceanic (Crawford 2000).

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1.2. Vegetation zonation in the Scottish mountains

It has long been recognised that the vegetation of the Scottish Highlands can

be described according to altitudinal zonation (Smith 1900, Smith 1911,

Matthews 1937, Watt & Jones 1948, Poore & McVean 1957, McVean &

Ratcliffe 1962, Burnet 1964, Ratcliffe & Thompson 1988, Pearsall 1989,

Brown et al. 1993). For example zones have been classified either side of the

potential Scottish tree-line with the montane zone above and the submontane

zone below (Brown et al. 1993). The montane zone is also referred to as the

“alpine” or “arctic-alpine” zone by a number of authors (Thompson & Brown

1992, Thompson et al. 1995, Nagy 1997, Gordon et al. 1998, Gordon et al.

2002 etc.). Horsfield and Thompson (1996) simplify this by recommending

that, in Scotland, the area below the potential tree-line be referred to as sub

alpine and forest zone habitats (these are the equivalents of the European

Sub Alpine and Forest zones) and above the tree-line as alpine habitats

(equivalent to the European Low and Middle Alpine zones), see Figure 1.2.

The most obvious vegetation zone boundary in mountain areas, which even

the lay person can recognise easily, is the tree-line. However, the ecological

definition of what constitutes a tree-line is rather more complex (see Körner

(1998) for a more detailed discussion of this point). In the context used here,

the tree-line can be defined as the upper altitudinal boundary at which a

forest or patches of forest grows, or in most of Scotland, would grow if it still

existed. At the upper edge of this boundary most species of tree show

stunted deformed growth, often referred to using the German term

krummholz (Grace 1989).

In Scotland, Grace (1997) estimates that the potential tree-line is at its

highest at ca. 620 m a.s.l. at Creag Fhiaclach (NH895055) in the Cairngorms,

although other authors suggest it maybe higher (Pears 1967, Pears 1968,

Horsfield & Thompson 1996). It declines to c. 520 m a.s.l. in the north west

Highlands and is considered to be close to sea level in the most exposed

areas of the northwest coast (Brown et al. 1993). In global terms this is an

anomalously low tree line and is thought to be mainly due to the harsh wind

8

climate experienced in the Scottish uplands (Grace 1989, 1990, 1997, Grace

& Norton 1990, Körner 1998).

Figure 1.2. Diagram indicating the altitudinal ranges of Scottish vegetation zones, these are lower in the north-west and far north where exposure is higher (on the left), and at higher altitudes in the east (on the right). Continental European terms are given on the left and Horsfield and Thompson’s (1996) recommended terms for Scotland on the right.

Above the potential tree-line in Scotland, vegetation is dominated by dwarf

shrubs which become increasingly prostrate due to wind clipping. At c. 800 m

a.s.l. there is a transition from wind clipped Calluna heath to montane heath,

and towards the summits Racomitrium heath can become dominant (referred

to as the Upper Arctic-Alpine Zone by Poore & McVean 1957). On north,

northeast or southeast facing slopes late laying snow beds can be found as

low as 800 m a.s.l. (Watson et al. 1994). Within these there are broad

vegetation types, some of which can be considered to be at risk from climatic

change, in particular snow bed vegetation.

Most authors (Poore & McVean 1957, McVean & Ratcliffe 1962, Burnet

1964, Pearsall 1989, Thompson & Brown 1992, Brown et al. 1993,

9

Thompson et al. 1995, Nagy 1997, Gordon et al. 1998, Gordon et al. 2002

etc.) note that these vegetation zones occur at lower altitudes in Scotland

than their continental European equivalents. This is probably due to the

oceanic nature of the Scottish climate, which is at the extreme end of several

European environmental gradients (Nagy 1997). The Grampian mountains,

which are the most continental area of the Scottish Highlands, are “highly

oceanic on the European scale” (Nagy 1997; see also Figure 1.1). The

Scottish winters are milder and wetter, the summers are cooler and wetter,

and the wind climate is also far harsher than in most of continental Europe.

The only mountain area of northern Europe which experiences a similarly

oceanic climate to Scotland is the south west coast of Norway. Therefore, the

nearest equivalent European vegetation zones to those in Scotland can be

found in Norway. The Scottish submontane zone is the equivalent of the

Norwegian subalpine forest zone, and the montane zone above the potential

Scottish tree-line is the equivalent of the Norwegian lower and middle alpine

zones (Brown et al. 1993).

1.3. Description of climate in the Scottish Highlands

Scotland has a maritime or oceanic climate and is therefore described as

having high oceanicity or low continentality. As stated above continentality is

a measure of how the climate of an area is affected by its remoteness from

the oceans and oceanic air (Conrad 1964). By comparison oceanicity is the

effect of maritime influences on a climate (Allaby 1994). Oceanic climates are

characterised by having a low annual temperature range, whereas

continental climates have a wide annual temperature range. This can have

important ecological effects, as plants are highly sensitive to the effects of

this oceanicity, such as changes in temperature and rainfall gradients, the

length and variability of the growing season (Crawford 2000).

Added to this are the climatic effects due to altitudinal gradients, which in

Scotland are particularly steep and can have significant effects even at

modest altitudes (Harrison & Kirkpatrick 2001). Scotland has a very high

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lapse rate (the rate at which temperature drops as altitude is gained) which is

strongly influenced by oceanic effects (Pepin 2001) so that lapse rates

change at a different rate to those in more continental areas. In Scotland

lapse-rates are typically 10.0-10.5˚C km-1, whereas in continental areas

lapse-rates are typically 6.0˚C km-1 (Harding 1978, Harrison 1994, Harrison

1997). Also, the relationship between the levels of summer warmth (often

expressed in degree days) and altitude is not linear (Parry 1976), and even

small changes in temperature can have a larger effect in the uplands than in

the lowlands. These are issues which should be borne in mind when

considering the potential effects of climatic change in Scotland.

1.4. Climate change

1.4.1. Global climate change

There is increasing evidence that the global climate is changing, with a global

temperature rise of about 0.6°C at the Earth surface over the last 100 years

(Houghton et al. 1996, IPCC 2000, IPCC 2001, Jones et al. 2001). Much of

this change has been attributed to increasing concentrations of greenhouse

gases, such as carbon dioxide (CO2), produced by human activities. A

number of models have been developed to predict possible climate change

scenarios according to relative levels of greenhouse gas emissions (Collins

et al. 2001, Durman et al. 2001, Hulme & Jenkins 1998, Hulme et al. 2002,

Mitchell et al. 1999, Jones & Reid 2001, Wood et al. 1999). See Figure 1.3.

These models predict a possible mean rise in global temperatures of

between 1.5°C and 5.8°C over the next century (IPCC 2001).

11

Figure 1.3. Reconstructed and recorded past temperatures, and modelled future temperature for the Northern Hemisphere shown as anomalies from a 1961 – 1990 temperature average. The reconstructed temperatures have been estimated from “proxy” climate indicators such as tree: rings, corals, ice cores, lake and ocean sediments, borehole measurements, historical documents and glacier moraines etc. (IPPC 2001) (figure taken from Hulme et al. 2002).

1.4.2. Predicted climate change scenarios for Scotland

The most recent predictions suggest that the average annual temperature in

the Scottish Highlands may increase by between 1°C and 2°C by 2050

(Harrison et al. 2001, Hulme & Jenkins 1998, Hulme et al. 2002). Using a

number of climate models developed by the Hadley Centre for Climate

Prediction and Research (Hulme et al. 2002), the UK Climate Impacts

Programme (UKCIP) has developed possible future climatic scenarios for the

UK (UKCIP02). These are based on the estimated impacts of global

emissions scenarios resulting from alternative paths of world development

(Hulme et al. 2002). One major uncertainty in the climate models is the fate

of the “thermohaline circulation” (THC), i.e. the Gulf Stream, which some

Showing departure in temperature from 1961 – 90 average (deg C) ranging from -1 to 7 on Y axis and years from 1000 to 2100 (AD) on X axis.

12

models predict may weaken or shut down entirely in the near future. Should

this occur, northwest Europe could experience cooling by up to 5°C within

the space of a few decades (Hulme et al. 2002).

The UKCIP scenario predictions have a 50 km grid resolution, but there are

still large uncertainties associated with these predictions. UKCIP02 does give

an assessment of confidence in its predictions, however it should be noted

that these confidence levels are qualitative and not quantitative (Hulme et al.

2002). In the following pages, the high emissions scenario for the 2050s was

used where available, this is also similar to the medium emission scenario for

the 2080s.

Temperature

As shown in Figure 1.4, the predicted increases in average daily

temperatures in Scotland over the next 50 years are by up to +1.63°C in

winter and + 2.31°C in summer using the UKCIP high emissions scenario.

The high emissions scenario is based on there being an increase in

concentration of CO2 to double that of levels of pre-industrial levels by 2045.

Unpredictable or uncertain changes, such as volcanoes or solar changes in

radiation, were not considered. The UKCIP give these predictions a high

relative level of confidence.

13

Figure 1.4. Predicted changes in daily mean temperatures (˚C) for Scotland over the next 50 years, showing an increase of up to 2.3°C by the 2050s (source: UKCIP).

Precipitation

Seasonal predictions for relative changes in precipitation in Scotland are

shown in Figure 1.5. These show a predicted decrease in summer rainfall of

up to 24% in the high emission scenario by the 2050s. Winter precipitation is

predicted to increase by up to 22% by 2050, with possibly the largest

increase in the east. UKCIP’s relative confidence level for the wetter winters

is given as “high”, but the relative confidence level for the drier summers is

given as “medium”. This is one of the changes from the UKCIP98 which was

predicting wetter summers.

Winter snowfall (Figure 1.6) is predicted to decrease in Scotland by the

2050s by up to 65%, with the largest decrease in the east. The UKCIP gives

a relatively high confidence level for “significant” declines in snowfall

everywhere in the UK.

14

Figure 1.5. Predicted percentage changes in precipitation for Scotland over the next 50 years, showing a decline of up to 24% in summer and an increase of up to 22% in winter in the high emission scenario. (source UKCIP).

Figure 1.6. Predicted percentage changes in average winter snowfall for Scotland in the 2050s showing a reduction of up to 65% using the UKCIP high emissions scenario. (source: UKCIP).

15

Wind speed

The predicted changes in daily mean wind speed for Scotland by the 2080s

are very small, between ± 3% for the annual average with slight increases in

spring and slight decreases in summer and autumn, while winter remains

within the “natural” variability (UKCIP02). The authors emphasize that, due to

inconsistency between different models as well as the physical

representation within the core model (HadRM3), they are unable to attach

any level of confidence to the predictions of wind speed (Hulme et al. 2002).

Diurnal temperature range

Diurnal temperature range, the difference in temperature between day and

night, is predicted to increase in Scotland by up to 1°C in summer by the

2080s, with a slight decrease in winter and smaller increases in other

seasons. UKCIP02 give a low level of confidence for this prediction.

1.5. Approach, hypotheses and thesis plan

Given the lack of suitable historical botanical surveys which could be used to

investigate the potential changes in Scottish mountain vegetation due to

global climate change over the past century, there is a need to find a different

approach. A two pronged approach was taken:

Firstly, it was necessary to collect information on the potential for seed to

move from lower vegetation zones to higher levels under predicted climate

change. Secondly, it was necessary to determine if an increase in mean

temperature, as predicted by the climatic models, would affect the strength of

interactions between plants, i.e. any change in the level of competition.

16

1.5.1. Hypotheses

To investigate the dispersal limitations of mountain plant species, the

following questions need to be addressed:

• Is there sufficient upward seed movement to enable an upward

migration of plants?

• Is there seed in the seedbank which could germinate and colonise

higher vegetation zones?

• Has the nature of the seedbank changed over recent decades?

To investigate the competitive interactions between plants at higher

temperatures, the following questions were posed:

• Would an increase in temperature change the balance from facilitation

(potential advantages of having neighbours, e.g. shelter, outweighing

the disadvantages of increased competition) to competition at high

altitudes?

• Is temperature a more important ecological factor than wind in the

Scottish alpine zone?

1.5.2. Brief outline of experimental approach – layout structure

In order to address the questions posed above with regard to the effects of

potential climatic change on mountain vegetation in Scotland, the following

two studies were undertaken.

Chapter 2: Potential for plants to move in a changing climate – A study of

mountain vegetation and seed dispersal.

To investigate the dispersal limitations of mountain plant species, soil cores

were taken at 50 m altitudinal intervals along four transects in the Grampian

mountains of Scotland, to measure the seed bank present. Current seed rain

17

was recorded at the same sites, using pitfall seed traps, over a two year

period. A vegetation survey was also carried out at all trapping stations, to

determine the current surface vegetation present. Current seed rain, surface

vegetation and the seed bank were compared to seek evidence for a

potential shift in vegetation.

Chapter 3: Effects of environmental manipulation of local climate on plant

interaction.

To test whether an amelioration of harsh environmental conditions,

simulating the effects of predicted climate change, would alter the balance

between competition and facilitation in arctic/alpine plant communities, an

experiment was set up using passive warming devices. Open Top Chambers

(OTCs), simple wind shelters and control treatments were set up on Glas

Tulaichean in the Glenshee area at 1000 m a.s.l., using Carex bigelowii and

Alchemilla alpina as target plants. These two species where chosen as they

both occur at the field site, are easy to use, and are predicted to be

vulnerable to global climate change (Berry et al. 2002, Berry pers com.). The

target plants were transplanted into replicate environmental treatments, and,

within each environmental treatment, they were planted with as well as

without neighbours. The OTCs are designed to achieve a warming effect by

reduction of wind speed and by acting as solar traps, in the same way as a

greenhouse (Marion 1996). Measurements were taken of final above ground

biomass of the target plants, as well as environmental variables (soil and air

temperature, wind speed and direction, soil moisture and PAR), to determine

if the warming provided by environmental treatments would be sufficient to

increase levels of competition experienced by the target species, and

whether temperature was a more important ecological factor than wind.

Chapter 4: Discussion

The thesis integrates the results of these two pieces of work with the relevant

literature and in Chapter 4 discusses potential changes in community

structure due to migration and changes in competition. The potential effects

18

of the predicted changes in environmental factors on mountain plants and

vegetation zones are also discussed, and compared with the findings of the

two experimental studies. Possible improvements to work carried out and

recommendations for further research are suggested.

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